The Classic MacOS toolbox used reserved instructions ($A000--$AFFF were reserved in the 68000 instruction set) which invoked the interrupt handler. Tool calls

The Classic MacOS toolbox used reserved instructions ($A000--$AFFF were reserved in the 68000 instruction set) which invoked the interrupt handler. Tool calls covered everything from string comparisons to windows and graphics but also included undeniably OS level stuff like opening files and allocating memory. You could use a toolbox call to get a function's address and call it directly, but that was mostly used for patching the toolbox.